Monoterpenes are a class of terpenes that consist of two isoprene units and have the molecular formula C10H16. Monoterpenes may be linear (acyclic) or contain rings (monocyclic and bicyclic). Modified terpenes, such as those containing oxygen functionality or missing a methyl group, are called monoterpenoids. Monoterpenes and monoterpenoids are diverse. They have relevance to the pharmaceutical, cosmetic, agricultural, and food industries. Biosynthesis Monoterpenes are derived biosynthetically from units of isopentenyl pyrophosphate, which is formed from acetyl-CoA via the intermediacy of mevalonic acid in the HMG-CoA reductase pathway. An alternative, unrelated biosynthesis pathway of IPP is known in some bacterial groups and the plastids of plants, the so-called MEP-(2-methyl-D-erythritol-4-phosphate) pathway, which is initiated from C5 sugars. In both pathways, IPP is isomerized to DMAPP by the enzyme isopentenyl pyrophosphate isomerase. Geranyl pyrophosphate is the precursor to monoterpenes (and hence monoterpenoids). Elimination of the pyrophosphate group from geranyl pyrophosphate leads to the formation of acyclic monoterpenes such as ocimene and the myrcenes. Hydrolysis of the phosphate groups leads to the prototypical acyclic monoterpenoid geraniol. Additional rearrangements and oxidations provide compounds such as citral, citronellal, citronellol, linalool, and many others. Many monoterpenes found in marine organisms are halogenated, such as halomon. Main examples Bicyclic monoterpenes include carene, sabinene, camphene, and thujene. Camphor, borneol, eucalyptol and ascaridole are examples of bicyclic monoterpenoids containing ketone, alcohol, ether, and bridging peroxide functional groups, respectively. Umbellulone is another example of bicyclic monoterpene ketone. Natural roles Monoterpenes are found in many parts of different plants, such as barks, heartwood, bark and leaves of coniferous trees, in vegetables, fruits and herbs. Essential oils are very rich in monoterpenes. Several monoterpenes produced by trees, such as linalool, hinokitiol, and ocimene have fungicidal and antibacterial activities and participate in wound healing. Some of these compounds are produced to protect the trees from insect attacks. Monoterpenes are emitted by forests and form aerosols that are proposed to serve as cloud condensation nuclei (CCN). Such aerosols can increase the brightness of clouds and cool the climate. Many monoterpenes have unique smell and flavor. For example, sabinene contributes to the spicy taste of black pepper, 3-carene gives cannabis an earthy taste and smell, citral has a lemon-like pleasant odor and contributes to the distinctive smell of citrus fruits, and thujene and carvacrol are responsible for the pungent flavors of summer savory and oregano, respectively. Monoterpenes are considered allelochemicals. Uses Many monoterpenes are volatile compounds